Methods of preparing crosslinked acrylic polymer microparticles commonly referred to as microgel particles are known in the art. One such method is disclosed in commonly-assigned copending application Ser. No. 296,700, filed Oct. 11, 1972, now U.S. Pat. No. 3,880,796, in the name of Roger M. Christenson et al. In this method, a non-aqueous polymer dispersion is prepared by polymerizing an ethylenically unsaturated monomer containing hydroxyl groups in the presence of (1) a dispersing liquid which is a solvent for the monomer but in which the resultant polymer is insoluble and (2) a dispersion stabilizer. The resultant non-aqueous polymer dispersion produced by this method consists of a major proportion of uncrosslinked acrylic polymer particles and a minor proportion (e.g., 10 percent by weight or less) of crosslinked acrylic polymer particles (i.e., microgel particles). Accordingly, in this method, it is necessary to separate the microgel particles from the uncrosslinked polymer particles. This is accomplished by the addition to the dispersion of an active solvent for the uncrosslinked polymer particles, thereby converting the dispersion to essentially a solution, but for the presence of the insoluble microgel particles. The microgel particles are then separated from the bulk of the polymer by conventional means such as centrifuging, filtering, and the like.
The above process, while advantageous in some respects, has several serious disadvantages. Thus, as will be apparent, the microgel particles are an incidental by-product of the non-aqueous dispersion process and therefore the yield is relatively low (e.g., 5 to 10 percent by weight or less). Moreover, because of this factor, it is necessary to separate the microgel particles from a dispersion which contains a major proportion of uncrosslinked acrylic polymer particles by dissolving the uncrosslinked polymer particles with an active solvent.
Still another method for producing microgel particles is disclosed in British Pat. No. 967,051 to Bullitt et al, dated Aug. 19, 1964. In this method, microgel particles are prepared by forming an aqueous emulsion of monoethylenic unsaturated monomer and a crosslinking monomer containing at least two ethylenic double bonds, heating the emulsion to a temperature of about 40.degree. to 100.degree. C. until the reaction is substantially complete to yield a microgel and during the reaction adding an agent to inhibit the formation of high molecular weight substantially uncrosslinked material. The inhibiting agent as disclosed in Bullitt et al can be an active solvent for the monomers or a chain transfer agent. This method has several disadvantages. Thus, the method utilizes conventional emulsion polymerization techniques requiring careful control of the process to prevent settling and the like. Further, the use of crosslinking monomers containing at least 2 ethylenic double bonds (e.g., divinyl and diacrylate monomers) has been found to lead to flocculation problems in relatively high solids level (i.e., 40 percent by weight or higher) microgel particle dispersions. Finally, this method requires the additional step of adding a water-immiscible solvent or chain transfer agent to the reaction mixture.